Conventionally, an apparatus has been known which transmits light from one side to the other side of a finger tip and detects changes in the transmittance (reflectance) of the transmitted light, for examining the amount of blood flowing in the finger. A resultant detected signal is then processed and the pulse rate, blood pressure and the like are evaluated by calculation. One example of a photosensor for use in such an apparatus is disclosed in Japanese Utility Model Laying-Open No. 60-158803.
FIG. 1 is a perspective view of a conventional photosensor, and FIG. 2 is a cross-sectional view of the photosensor of FIG. 1 attached to a finger.
Referring to FIG. 1, a conventional photosensor 8 transmits light from one side to the other side of a finger for detecting changes in the transmittance of the light. A light emitting element 2 and a light receiving element 3 are disposed with a predetermined spacing therebetween corresponding to the size of the finger, on an easily bendable film substrate 1. A transparent and easily bendable transparent film 6 is placed or attached to film substrate 1 to cover light emitting element 2 and light receiving element 3.
Referring to FIG. 2, the photosensor 8 is used by winding it around a finger 7 to interpose the tip of finger 7 between light emitting element 2 and light receiving element 3. A fixing tape 30 (e.g., a so-called magic tape) is attached around photosensor 8 wound around finger 7. Photosensor 8 is securely fixed on finger 7 by winding fixing tape 30 around photosensor 8 and overlapping the surface of one end of fixing tape 30 and the reverse surface of the other end thereof. When power is supplied from a signal processing unit (not shown) through a connecter 5 to a lead 4, light emitting element 2 emits light. The emitted light is transmitted through finger 7 and directed onto light receiving element 3 which receives light and transmits a resultant signal through lead 4 and connector 5 to the signal processing unit. The signal processing unit detects a change in the transmittance of the light provided at this time, processes a detected signal thereof and then evaluates the pulse rate and blood pressure value by calculation.
In general, a light emitting diode is used as the light emitting element 2 employed in the above-described photosensor 8. The light emitting diode has, however, an undesirable property in that its output power and a wavelength of light emitted from the diode vary depending on the ambient temperatures. If photosensor 8 is attached to a living body, e.g. finger 7, then finger 7 becomes ischemic or hemostatic, resulting in a decrease in the body temperature of finger 7 or in an increase in the body temperature thereof due to an increase in blood pressure. Thus, the ambient temperature of light emitting element 2 varies and its output power and its measured wavelength of light vary accordingly. However, it is desirable that the output power of light emitting element 2 and the wavelength of the light emitted from light emitting element 2 are kept constant in order to accurately measure a pulse rate or a blood pressure value.
In another disclosure a light emitting diode has been proposed which is shown in FIGS. 3A and 3B as the one satisfying the above requirements Japanese Patent Application No. 1-116757 FIG. 3A is a plan view of the kanda light emitting diode, and FIG. 3B is a side view of the diode of FIG. 3A. With reference to FIGS. 3A and 3B, a light emitting diode 10 includes two LED chips LED.sub.1 and LED.sub.2 disposed on a substrate 12. A photodiode PD is provided in the vicinity of LED chips LED.sub.1 and LED.sub.2. Photodiode PD directly receives light emitted from chips LED.sub.1 and LED.sub.2.
Photodiode PD detects a change in the amount of the light emitted from the chips LED.sub.1 and LED.sub.2 in accordance with a change in the ambient temperatures. A transparent epoxy resin 13 is applied onto substrate 12 to cover the chips LED.sub.1 and LED.sub.2 and the photodiode PD.
An operation will now be described. Photodiode PD, which is a light receiving element provided separately from the light receiving element of FIG. 1, is disposed near the chips LED.sub.1 and LED.sub.2. Photodiode PD detects a change in the amount of the light from the light emitting diode in accordance with a change in the ambient temperature. A current flowing through the chips LED.sub.1 and LED.sub.2 is controlled so as to correct the change in the amount of the light. This feature makes it possible to keep the output power of and the wavelength of the light emitted by the chips LED.sub.1 and LED.sub.2 constant and thus to obtain an accurate information as to a living body.
The light emitting diode thus structured has, however, room for further improvement.
With reference to FIG. 4, since the light emitting diode 10 is used in contact with a living body 20, there are three types of light beams: a light beam designated by the dotted line 1 which is emitted from the chip LED.sub.2 and is directly incident on photodiode PD; a light beam designated by the dotted line 2 which undergoes a total reflection from an inner surface of epoxy resin 13 and enters into photodiode PD; and a light beam designated by the dotted line 3 which once enters into living body 20 and is then scattered or reflected by living body 20, entering into photodiode PD. The amount of the scattered light or the reflected light denoted by the dotted line 3 is not constant. Accordingly, there occurs an error in a feedback apparatus for monitoring the amount of the light emitted from the chips LED.sub.1 and LED.sub.2 to maintain a constant light output becomes impossible.